A gear to be used for a transmission for a vehicle, for example, has been required to have high pitting resistance and bending strength, and in order to meet such a requirement, a carburizing treatment has been widely performed until now as a method of strengthening a steel member such as a gear. Further, with the aim of further improving the pitting resistance, there has been proposed an invention related to achievement of high strength by a carbonitriding treatment (Patent Document 1). On the other hand, with regard to a planetary gear, due to its engagement degree being high, an effect of tooth profile accuracy (strain) on gear noise has been large, and particularly, an internal gear has had a problem of being likely to be strained due to being thin and large in diameter. Thus, there has been also proposed an invention related to a gas nitrocarburizing treatment causing less strain of a steel member and also causing small variations in strain (Patent Document 2).